1. Field of the Invention
The present invention relates to a semiconductor device which reduces soft errors caused by fast neutrons.
2. Description of the Related Art
In an SRAM (Static Random Access Memory) and DRAM (Dynamic Random Access Memory) as semiconductor memories, a phenomenon in which held data is naturally destroyed occurs. This phenomenon is called a “soft error”.
Known causes of soft errors are α-rays emitted from radioactive substances contained in materials such as solder used in a semiconductor device, and fast neutrons which come flying as cosmic rays.
Soft errors caused by α-rays can be avoided relatively easily by designing semiconductor elements beforehand, e.g., by reducing radioactive substances contained in a semiconductor device, or setting a film thickness such that no data destruction occurs even when α-rays enter. Also, the absolute amount of electron-hole pairs produced by a soft error caused by an α-ray is smaller than that of electron-hole pairs produced by a soft error caused by a fast neutron (to be described below). Soft errors caused by α-rays are easy to avoid in this sense, too.
On the other hand, a fast neutron which causes a soft error has almost no influence on a semiconductor device even when this fast neutron itself passes through Si (silicon). However, if a fast neutron collides with an Si atom in a semiconductor device to cause nuclear spallation, a secondary particle having an atomic number smaller than that of the collided Si atom is ejected, and electron-hole pairs are produced along the orbit of this secondary particle. That is, if this secondary particle penetrates by this p-n junction, electron-holes pairs produced along the orbit of the secondary particle move under the influence of a bias applied to the p-n junction, in the same manner as in soft errors caused by α-rays. As a consequence, these electron-hole pairs produce a noise current to cause an operation error of an element. This problem is more serious than soft errors caused by α-rays since the order of the absolute amount of electron-hole pairs generated in the process is large.
References pertaining to soft errors are patent references 1, 2, and 3 below.
[Patent Reference 1]
Jpn. Pat. Appln. KOKAI Publication No. 11-354690
[Patent Reference 2]
Jpn. Pat. Appln. KOKAI Publication No. 8-330478
[Patent Reference 3]
Jpn. Pat. Appln. KOKAI Publication No. 9-260427